While the renin-angiotensin system and its role in cardiovascular disease, in particular essential hypertension, has been investigated in detail, angiotensnogen, the precursor of angiotensin, has escaped critical examination. A radioimmunoassay (RIA) is proposed for the assay of this plasma protein. A penultimate moiety will be selectively cleaved by chemical means from angiotensinogen and used as an antigen. Antibodies to this antigen will be produced in experimental animals, and this antibody will be utilized in the RIA following established procedures. The concentrations of angiotensinogen in plasma will be evaluated both in health and in certain clinical cases, in particular essential hypertension and in pregnancy.